Is there a simple way to get image dimensions in Ruby?

Question

I\'m looking for an easy way to get width and height dimensions for image files in Ruby without having to use ImageMagick or ImageScience (running Snow Leapard).

Answer 1

Here's a version of the JPEG class from ChristopheD's answer that works in both Ruby 1.8.7 and Ruby 1.9. This allows you to get the width and height of a JPEG (.jpg) image file by looking directly at the bits. (Alternatively, just use the Dimensions gem, as suggested in another answer.)

class JPEG
  attr_reader :width, :height, :bits
  def initialize(file)
    if file.kind_of? IO
      examine(file)
    else
      File.open(file, 'rb') { |io| examine(io) }
    end
  end
private
  def examine(io)
    if RUBY_VERSION >= "1.9"
      class << io
        def getc; super.bytes.first; end
        def readchar; super.bytes.first; end
      end
    end
    class << io
      def readint; (readchar << 8) + readchar; end
      def readframe; read(readint - 2); end
      def readsof; [readint, readchar, readint, readint, readchar]; end
      def next
        c = readchar while c != 0xFF
        c = readchar while c == 0xFF
        c
      end
    end
    raise 'malformed JPEG' unless io.getc == 0xFF && io.getc == 0xD8 # SOI
    while marker = io.next
      case marker
        when 0xC0..0xC3, 0xC5..0xC7, 0xC9..0xCB, 0xCD..0xCF # SOF markers
          length, @bits, @height, @width, components = io.readsof
          raise 'malformed JPEG' unless length == 8 + components * 3
        # colons not allowed in 1.9, change to "then"
        when 0xD9, 0xDA then  break # EOI, SOS
        when 0xFE then        @comment = io.readframe # COM
        when 0xE1 then        io.readframe # APP1, contains EXIF tag
        else                  io.readframe # ignore frame
      end
    end
  end
end